Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Dangerous avalanche conditions exist. You could trigger avalanches big enough to bury people on many sheltered and wind-affected slopes. You could trigger avalanches remotely—from gentler terrain above, below, and to the sides of steep slopes. To reduce your risk, you can stick to lower-angled terrain that is not directly below large avalanche paths. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    The calendar says April, but we're still dealing with a mid-winter-style Persistent Slab problem (watch this video). This problem is by far the most likely way to "find trouble" today. We have two distinct problems to consider:

    1. "Fresh" Slabs: Sunday, a SAC forecaster remotely triggered a large avalanche in this zone (Banner Summit video), and an observer heard 2 natural avalanches release near Copper Mtn. You could trigger 1-2 foot thick slides on middle and upper elevation slopes. Avalanches are most likely on slopes approaching or over about 35 degrees where crust + facet combinations or small facets exist in the upper 2 feet of the snowpack. Watch for snowpack cracking or collapsing, clear signs the snowpack is unstable. 
    2. Thicker, Older Slabs: While not likely, the possibility of triggering a massive 3-6 foot thick avalanche remains on the table. These hard slabs also rest on weak facets and crust + facet combinations. The most recent triggered avalanche involving this snow occurred Friday in the Frenchman Ck drainage near Smiley Creek when riders remotely triggered a slide that broke 4-5 feet deep and 800+ feet wide (photo at right/above). The riders were at least 500 feet away on the ridgeline above when they triggered the slide. Triggering an avalanche involving this deeper weak layer is less likely than the thinner "fresh slabs", but the consequences would be devastating. Don't expect to see signs of instability associated with these thick, dense slabs. 

    The multiple persistent slab problems are challenging to manage. You may or may not see obvious signs of instability on unstable slopes, depending on the depth of the weak layer. You may be able to trigger slides from long distances away. Slides could break wider than you expect. Tracks on a slope are not a great indicator of stability. Unfortunately, the best management strategy is avoidance; staying off of slopes steeper than about 35 degrees will reduce your risk, but it won't eliminate it. 

    ADDITIONAL DISCUSSION: You may encounter wind slabs in exposed upper elevation terrain. Any wind slabs that you encounter may be perched above weak facets and/or crusts, making avalanches easier to trigger and wider than you'd expect. Look for wind slabs near ridgelines and beneath cornices. Surface textures like dunes, ripples, or pillows are clues that the wind has been at work. Cracks shooting out from your skis or machine are clear indicators that you've found an unstable slab. 

    Cornices are unusually large this year. Give these unpredictable beasts plenty of space by traveling far from the edge, staying where you can easily feel dirt or rocks below your skis or sled. Large, falling cornice chunks and/or relatively small wind slabs moving downhill could trigger the larger, more dangerous Persistent Slab avalanches discussed above.

    (3/31/2023)

    Crown of a very large avalanche that was remotely triggered by riders from at least 500 feet away on the ridgeline above. It broke 4-5 feet deep and 800+feet wide. This slope faces W at 9,300' in the Frencman's drainage near Smiley Creek. You can clearly see the two sets of persistent weak layers in our snowpack in this photo. 

Avalanche Discussion

The cold, sputtery weather in the past 48 hours is preventing our Persistent Slab problem from fading off into the sunset.   A quick recap of some important observations from the past couple of days:

  • SAC forecasters remotely triggered two avalanches (Banner Summit video, Titus Ridge observation).
  • A regular observer reported multiple, large, natural avalanches released around noon Sunday near Copper Mtn on E aspects near 8500'.
  • Several medium-sized slab and loose snow avalanches released naturally on Sunday in the Wood River Valley south of Ketchum. All the slides were in west-facing, wind-loaded avalanche paths that were baking in the afternoon sun.
  • A regular observer reported several impressive recent natural avalanches in the Pioneer Mtns (observation).
  • SAC forecasters and observers reported dozens of recent large natural avalanches, including impressive slides on Galena Peak and farther south in the Boulder Mtns (observation with photos). 

Are there patterns in the recent activity? Many recent avalanches appeared to involve "prime time" slope angles of 35-45 degrees. Recent avalanches failed on many aspects: sunny S-SW slopes, slopes on the "margins" with partial shade and sun (W-NW and SE-E), and shadier aspects (NW-N-NE). Most of the recent activity appeared to involve crust + facet combinations in the upper 2 feet of the snowpack, but some slides were over 4 feet thick. Many of the recent slides were in relatively sheltered terrain, and many were over 500' wide. 

What can we do with that information? On slopes steeper than about 35 degrees, there's a decent chance you'll trigger a slide big enough to bury you and an outside chance the slide will be big enough to damage or destroy a house. Consider what's above you if you're sticking to gentler slopes; you could trigger avalanches from flat or mellow terrain below steep avalanche starting zones.

A note on wet avalanches: Adding to the complexity is the potential for wet avalanches. While they won't be huge, they could trigger the Persistent Slabs discussed above or carry you where you don't want to go. Muted sunshine and afternoon heating could cause natural, surficial wet loose slides involving the recent snowfall, especially on SE-S-SW-W aspects when they are directly facing the sun. In lower elevation terrain where the snowpack is thin, wet loose avalanches could gouge to the ground. Watch for rollerballs and pinwheels, signs the surface snow is becoming moist and unstable. Steer clear of steep slopes where you find a wet, mushy snowpack.  

Regioni coperte